The Marxism of the Second International (1889–1814/1918)
This period comprises the debates conducted during the first decades following Marx's death. These debates were already characterised by intense and open arguments on the "correct" interpretation of his theories and on how to practically implement his critique of society. Its most well-known exponents were typically both recognised theorists and political activists.
They include, among others, Karl Kautsky, Rosa Luxemburg, Rudolf Hilferding, Lenin, Wilhelm and Karl Liebknecht, August Bebel, Clara Zektin and Alexandra Kollontai. Less well-known protagonists such as Herman Gorter, Tom Mann, Daniel De Leon, Georgi Plekhanov or Antonio Labriola should, however, not be forgotten. Important contributions, organised by author, can be found in the above-mentioned Marxist Internet Archive, as well as in the anthology Stimmen der proletarischen Revolution ("Voices of the Proletarian Revolution"). The German edition of Lenin's works, published in the German Democratic Republic from 1961 onwards, is all online, in PDF format, including the supplementary volumes and index.
The Historico-Critical Dictionary of Marxism, published in German from 1983 onwards, provides helpful supplementary information on specific aspects of this period of Marxist debates and political movements – see for example the entry on "Leninism".